Class delegation with by keyword in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delegate the interface implementation using the 'by' keyword.

Kotlin

interface Printer {
    fun printMessage()
}

class ConsolePrinter : Printer {
    override fun printMessage() {
        println("Printing to console")
    }
}

class Delegator(private val printer: Printer) : Printer [1] printer

fun main() {
    val printer = ConsolePrinter()
    val delegator = Delegator(printer)
    delegator.printMessage()
}

Drag options to blanks, or click blank then click option'

Aimplements

Bextends

Cby

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' or 'extends' which are Java keywords, not Kotlin delegation keywords.

Using ':' alone without 'by' does not delegate implementation.

In Kotlin, the 'by' keyword is used for class delegation to delegate interface implementation to another object.

2fill in blank

medium

Complete the code to delegate the 'calculate' function to the CalculatorImpl class.

Kotlin

interface Calculator {
    fun calculate(x: Int, y: Int): Int
}

class CalculatorImpl : Calculator {
    override fun calculate(x: Int, y: Int): Int {
        return x + y
    }
}

class DelegatedCalculator(private val calc: Calculator) : Calculator [1] calc

fun main() {
    val calculator = CalculatorImpl()
    val delegated = DelegatedCalculator(calculator)
    println(delegated.calculate(5, 3))
}

Drag options to blanks, or click blank then click option'

Aimplements

Bby

Cextends

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using Java keywords like 'implements' or 'extends' instead of Kotlin's 'by'.

Omitting the delegation keyword causes a compilation error.

The 'by' keyword delegates the interface implementation to the provided object in Kotlin.

3fill in blank

hard

Fix the error in the delegation syntax to correctly delegate the 'speak' function.

Kotlin

interface Speaker {
    fun speak()
}

class EnglishSpeaker : Speaker {
    override fun speak() {
        println("Hello")
    }
}

class Person(private val speaker: Speaker) : Speaker [1] speaker

fun main() {
    val englishSpeaker = EnglishSpeaker()
    val person = Person(englishSpeaker)
    person.speak()
}

Drag options to blanks, or click blank then click option'

Aby

Bimplements

Cextends

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using Java keywords like 'implements' or 'extends' instead of 'by'.

Using ':' alone without 'by' causes syntax errors.

The correct Kotlin syntax for delegation uses the 'by' keyword to delegate interface implementation.

4fill in blank

hard

Fill both blanks to delegate the 'draw' function to the 'ShapeImpl' class.

Kotlin

interface Shape {
    fun draw()
}

class ShapeImpl : Shape {
    override fun draw() {
        println("Drawing shape")
    }
}

class ShapeDelegate(private val shape: Shape) : Shape [1] [2]

fun main() {
    val shapeImpl = ShapeImpl()
    val shapeDelegate = ShapeDelegate(shapeImpl)
    shapeDelegate.draw()
}

Drag options to blanks, or click blank then click option'

Aby

Bimplements

Cshape

Dextends

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' or 'extends' instead of 'by'.

Not specifying the object to delegate to.

The 'by' keyword delegates the interface implementation to the object 'shape'.

5fill in blank

hard

Fill all three blanks to delegate the 'play' function to the 'MusicPlayerImpl' class and override it in the delegator.

Kotlin

interface MusicPlayer {
    fun play()
}

class MusicPlayerImpl : MusicPlayer {
    override fun play() {
        println("Playing music")
    }
}

class CustomPlayer(private val player: MusicPlayer) : MusicPlayer [1] [2] {
    override fun play() {
        println("Custom player start")
        player.[3]()
        println("Custom player end")
    }
}

fun main() {
    val impl = MusicPlayerImpl()
    val custom = CustomPlayer(impl)
    custom.play()
}

Drag options to blanks, or click blank then click option'

Aby

Bplayer

Cplay

Dimplements

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' instead of 'by'.

Forgetting to call the delegated function inside the override.

Using wrong variable or function names.

Use 'by' to delegate, 'player' as the delegate object, and call 'play()' on it inside the override.

Practice

1. What does the by keyword do in Kotlin class delegation?

easy

A. It overrides all methods of a class.

B. It creates a new instance of a class.

C. It marks a class as abstract.

D. It lets a class reuse another class's implementation automatically.

Solution

1. Step 1: Understand the role of by keyword

   The by keyword in Kotlin is used for class delegation, which means one class can reuse the implementation of another class without rewriting the code.

2. Step 2: Identify what by does

   It automatically forwards calls to the delegated class, so the delegating class behaves like the delegated one.

3. Final Answer:

   It lets a class reuse another class's implementation automatically. -> Option D

4. Quick Check:

   Delegation = reuse implementation [OK]

Hint: Remember: by means reuse another class's code [OK]

Common Mistakes:

• Thinking by creates a new instance
• Confusing by with inheritance
• Assuming by overrides methods automatically

2. Which of the following is the correct syntax to delegate interface Printer to class ConsolePrinter in Kotlin?

easy

A. class MyPrinter : Printer by ConsolePrinter()

B. class MyPrinter implements Printer by ConsolePrinter()

C. class MyPrinter extends Printer by ConsolePrinter()

D. class MyPrinter delegate Printer to ConsolePrinter()

Solution

1. Step 1: Recall Kotlin delegation syntax

   In Kotlin, delegation uses the syntax: class ClassName : InterfaceName by instance.

2. Step 2: Match the correct syntax

   class MyPrinter : Printer by ConsolePrinter() matches this pattern exactly: class MyPrinter : Printer by ConsolePrinter().

3. Final Answer:

   class MyPrinter : Printer by ConsolePrinter() -> Option A

4. Quick Check:

   Syntax = class X : Interface by instance [OK]

Hint: Use colon and 'by' for delegation syntax [OK]

Common Mistakes:

• Using 'implements' or 'extends' keywords (Java style)
• Writing 'delegate' keyword which doesn't exist
• Missing colon before interface name

3. What is the output of this Kotlin code?

interface Logger {
    fun log(msg: String)
}

class ConsoleLogger : Logger {
    override fun log(msg: String) {
        println("Console: $msg")
    }
}

class AppLogger(logger: Logger) : Logger by logger

fun main() {
    val logger = AppLogger(ConsoleLogger())
    logger.log("Hello")
}

medium

A. No output

B. Console: Hello

C. Error: log() not implemented

D. Hello

Solution

1. Step 1: Understand delegation in AppLogger

   The class AppLogger delegates Logger interface to the passed logger instance, which is ConsoleLogger.

2. Step 2: Trace the log call

   Calling logger.log("Hello") calls ConsoleLogger.log, which prints "Console: Hello".

3. Final Answer:

   Console: Hello -> Option B

4. Quick Check:

   Delegation calls delegated method output [OK]

Hint: Delegated methods run from the delegated instance [OK]

Common Mistakes:

• Expecting just 'Hello' without prefix
• Thinking delegation requires manual override
• Assuming no output because of delegation

4. Identify the error in this Kotlin code using class delegation:

interface Speaker {
    fun speak()
}

class Human : Speaker {
    override fun speak() {
        println("Hello")
    }
}

class Robot(speaker: Speaker) : Speaker {
    speak()
}

medium

A. Missing delegation with 'by' keyword in Robot class.

B. Robot class should not implement Speaker interface.

C. Human class must not override speak().

D. speak() method call is correct as is.

Solution

1. Step 1: Check Robot class declaration

   Robot implements Speaker but does not delegate or implement speak() method itself.

2. Step 2: Identify missing delegation

   Robot should delegate Speaker to the passed speaker instance using by speaker to avoid error.

3. Final Answer:

   Missing delegation with 'by' keyword in Robot class. -> Option A

4. Quick Check:

   Delegation needs 'by' keyword [OK]

Hint: Delegate with 'by' or implement all methods [OK]

Common Mistakes:

• Calling method directly without delegation
• Forgetting to use 'by' keyword
• Assuming interface methods auto-implement

5. You want to create a class SmartSpeaker that delegates Speaker interface to a Human instance but adds a new method playMusic(). Which is the best way to do this?

hard

A. class SmartSpeaker : Human() {
fun playMusic() = println("Playing music")
}

B. class SmartSpeaker : Speaker {
val human = Human()
override fun speak() = human.speak()
fun playMusic() = println("Playing music")
}

C. class SmartSpeaker(human: Human) : Speaker by human {
fun playMusic() {
println("Playing music")
}
}

D. class SmartSpeaker(human: Human) : Speaker {
fun playMusic() = println("Playing music")
}

Solution

1. Step 1: Understand delegation with added methods

   Using by human delegates all Speaker methods to Human automatically.

2. Step 2: Add new method in delegating class

   We can add playMusic() directly in SmartSpeaker without affecting delegation.

3. Final Answer:

   class SmartSpeaker(human: Human) : Speaker by human { fun playMusic() { println("Playing music") } } -> Option C

4. Quick Check:

   Delegate with 'by' and add methods [OK]

Hint: Delegate with 'by' and add new methods in class body [OK]

Common Mistakes:

• Not delegating interface methods properly
• Extending class instead of delegating interface
• Forgetting to implement interface methods